Organic EL display device having host compound and phosphorescent luminous compound, and method of driving same

ABSTRACT

In an organic electroluminescence display device ( 30 ) comprising an organic EL element ( 26 ) having a structure wherein an organic luminescent medium ( 24 ) is sandwiched between a top electrode ( 20 ) and a bottom electrode ( 22 ), and a driving circuit ( 14 ) for driving the organic EL element ( 26 ), the organic luminescent medium ( 24 ) comprises a host compound and a triplet-related luminous compound and the driving circuit ( 14 ) applies a electric pulse voltage or pulse current having a frequency of 30 Hz or more and a duty ratio of 1/5 or less. In this way, it is possible to provide an organic EL display device which consumes a low electric power and has a long luminous life span, and a method for driving the same.

TECHNICAL FIELD

This invention relates to an organic electroluminescence display device(which may be referred to as an organic EL display device hereinafter),and a method for driving the same. More specifically, the inventionrelates to an organic EL display device which consumes a low electricpower and is capable of obtaining a long luminous life span, and amethod for driving the same.

The “EL” described in the present specification is an abbreviation of“electroluminescence”.

BACKGROUND ART

Hitherto, there have been known simple driving type organic EL displaydevices wherein an organic EL element having an organic emitting layersandwiched between electrodes is driven by means of an XY matrixelectrode structure, and the devices are disclosed in, for example,Japanese Patent Application Laid-Open (JP-A) Nos. 2-37385 and 3-233891.

In such a simple driving type organic EL luminous device, the so-calledline sequential scanning is performed. Therefore, in the case that thenumber of scan lines is several hundred, the instantaneous brightnessrequired is several hundred times observation brightness. Consequently,the following problems are caused.

-   (1) The driving voltage thereof is 2 to 3 times higher than that in    the case of direct current constant voltage. For this reason, the    luminescence efficiency lowers or the consumption power gets larger.-   (2) Since the quantity of flowing electric current instantaneously    becomes several hundred times larger, the organic emitting layer is    liable to deteriorate.-   (3) Since the electric current quantity is very large in the same    manner as in the (2), a drop in voltage in wiring of the electrodes    gets larger.

Therefore, in order to solve the problems which simple driving typeorganic EL luminous devices have, suggested is an active driving typeorganic EL luminous device having a thin film transistor, (which may bereferred to as a TFT hereinafter), so as to drive organic EL elements.

Such active driving type organic EL luminous devices havecharacteristics that the driving voltage thereof is largely lowered, theluminescence efficiency is improved and further the consumption powercan be reduced with compared to simple driving type organic EL luminousdevices.

However, in the case that an active driving type organic EL luminousdevice having such advantageous effects has a compound having the lightemitting property to which the triplet state contributes (hereinafterreferred to as triplet-related compound), for example, an organicluminescent medium containing an iridium complex, a high luminescencebrightness is obtained whereas a problem that the luminous life span isshort is encountered. That is, in triplet-related compounds, the timewhen the molecules thereof are excited and relaxed is longer in the caseof compounds having the light property related to the singlet state.Hence, electric charge is stored inside, so that the balance betweenholes and electrons is liable to be destroyed. Thus, according to JapanJournal of Applied Physics, Vol. 38, pp. L1502–L1504 (1999), in the caseof an organic EL luminous device having an organic luminescent mediumcontaining an iridium complex, the luminescence efficiency thereof is avalue of 40 lumens/W or more under the condition that the luminescencebrightness is 500 cd/cm² but the half life thereof is a short time of200 hours or less.

Thus, the present inventors made eager investigation on theabove-mentioned problems. As a result, it has been found out that anorganic EL display device which consumes a low electric power and isdrivable for a long time can be provided by driving the device throughan appropriately-set driving circuit even if a combination of a hostcompound with a triplet-related luminous compound is used; accordingly,the display device can be applied to the field of flat panel displaysand others.

That is, an object of the present invention is to provide an organic ELdisplay device which consumes a low electric power and gives a less dropin luminescence brightness even when the device is driven for a longtime.

Another object of the present invention is to provide a driving methodcapable of driving such an organic EL display device at a lowconsumption power for a long time.

DISCLOSURE OF THE INVENTION

According to the present invention, provided is an organic EL displaydevice comprising an organic EL element having a structure wherein anorganic luminescent medium comprising a host compound and aphosphorescent luminous compound is sandwiched between a top electrodeand a bottom electrode; and a driving circuit for applying a electricpulse current or pulse voltage having a frequency of 30 Hz or more and aduty ratio of 1/5 or less so as to drive the organic EL element.

The organic EL display device having such a structure makes it possibleto make consumption power low even if a triplet-related luminouscompound is used, and further to make the luminous life span thereoflong.

In order to construct the present invention, it is preferable that thepresent invention has a driving circuit for applying the electric pulsevoltage or pulse current to cause the organic luminescent medium to emitluminescence, and subsequently applying a voltage (V2) in the directionreverse to that of the voltage (V1) of the pulse wave applied betweenthe electrodes of the organic electroluminescence element.

The organic EL display device having such a structure makes it possibleto make the luminous life span longer since electric charge storedinside can be removed even when a triplet-related luminous compound isused as the organic luminescent medium.

In order to construct the present invention, it is preferable that theorganic EL display device of the present invention has a driving circuitfor applying the voltage (V2) which is a smaller than the voltage (V1)of the pulse wave and is in the direction reverse to that of the voltage(V1).

The organic EL display device having such a structure makes it possibleto make the luminous life span still longer even when a triplet-relatedluminous compound is used.

In order to construct the present invention, it is preferable that thetriplet-related luminous compound is an organic metal complex.

The organic EL display device having such a structure makes it possibleto make consumption power lower.

In order to construct the present invention, it is preferable that theorganic metal complex comprises at least one metal selected from thegroup consisting of Ir, Pt, Pd, Ru, Rh, Mo, Re, Pb and Bi.

The organic EL display device having such a structure makes it possibleto make consumption power lower.

In order to construct the present invention, it is preferable that theorganic EL display device of the present invention comprises a holebarrier layer between the organic luminescent medium and the cathode.

The organic EL display device having such a structure makes it possibleto make consumption power lower even if a triplet-related luminouscompound is used, and further to make the luminous life span longer.

In order to construct the present invention, it is preferable that thehole barrier layer comprises a phenanthroline derivative.

The organic EL display device having such a structure makes it possibleto make consumption power lower even if a triplet-related luminouscompound is used, and further to make the luminous life span longer.

In order to construct the present invention, it is preferable that thedriving circuit comprises a thin film transistor for controlling theluminescence of the organic EL element.

The organic EL display device having such a structure makes it possibleto make consumption power lower even if a triplet-related luminouscompound is used, and further to make the luminous life span longer.

Another aspect of the present invention is a method for driving anorganic luminescence display device comprising an organicelectroluminescence element having a structure wherein an organicluminescent medium is sandwiched between a top electrode and a bottomelectrode, comprising applying a electric pulse current or pulse voltagehaving a frequency of 30 Hz or more and a duty ratio of 1/5 or less bymeans of a driving circuit, so as to drive the organicelectroluminescence element. Preferably, the electric pulse current isapplied.

By driving the organic EL display device in this way, a low consumptionpower is attained even when a triplet-related luminous compound is used,and further the luminous life span can be made longer.

In order to carry out the driving method of the present invention, it ispreferable that the driving circuit applies the electric pulse voltageor pulse current to cause the organic luminescent medium to emitluminescence, and subsequently applies a voltage (V2) in the directionreverse to that of the voltage (V1) of the pulse wave applied betweenthe electrodes of the organic electroluminescence element.

By driving the organic EL display device in this way, the luminous lifespan can be made longer since electric charges stored inside can beeffectively removed even when a triplet-related luminous compound isused.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of an organic EL display device of thepresent invention;

FIG. 2 is a graph showing a relationship between duty ratio and halflife;

FIG. 3 is a circuit diagram including a TFT;

FIG. 4 is a layout diagram including the TFT;

FIG. 5 is a timing chart at the time of applying pulse waves forluminescence;

FIG. 6 is a timing chart at the time of applying a reverse voltage;

FIG. 7 is a timing chart at the time of applying a reverse voltage;

FIG. 8 is a timing chart at the time of applying a reverse voltage; and

FIG. 9 is a timing chart at the time of applying a reverse voltage.

BEST MODES FOR CARRYING OUT THE INVENTION

Referred to the drawings, embodiments of the present invention arespecifically described hereinafter. The drawings referred to merelyillustrate the size and the shape of respective constituting elementsand the arrangement relationship therebetween schematically to such anextent that this invention can be understood. Accordingly, thisinvention is not limited to illustrated examples only. In the drawings,hatching representing a section may be omitted.

First Embodiment

As illustrated in FIG. 1, an organic EL luminous device of a firstembodiment is an organic EL display device 30 having an organic ELelement 26 wherein an organic luminescent medium 24 is sandwichedbetween a top electrode 20 and a bottom electrode 22, which are set overa supporting substrate 10, and a driving circuit 14 for driving theorganic EL element 26, wherein the organic luminescent medium 24comprises a host compound and a triplet-related luminous compound, andfurther the driving circuit 14 can apply an electric pulse voltage (V1)or pulse current having a frequency of 30 Hz or more and a duty ratio of1/5 or less, the voltage (V1) being applied between the electrodes ofthe organic EL element 26 at this time.

In FIG. 1, a TFT circuit is set up. However, this TFT circuit may beomitted. Organic EL display devices wherein a TFT circuit is set up areusually driven by direct current whereas the above-mentioned electricpulse voltage or pulse current is applied in the present invention.

About the embodiment of the organic EL luminous device of the presentinvention, constituting elements thereof, the driving method thereof,and others are described with reference to FIG. 1, and other figures.

1. Supporting Substrate

The support substrate in the organic EL display device, (which may bereferred to as the substrate hereinafter), is a member for supportingthe organic EL element, the driving circuit, and others. It is thereforepreferable that the substrate is excellent in mechanical strength anddimension stability.

Specific examples of such a substrate include a glass plate, a metalplate, a ceramic plate or a plastic plate (such as polycarbonate resin,acrylic resin, vinyl chloride resin, polyethylene terephthalate resin,polyimide resin, polyester resin, epoxy resin, phenol resin, siliconresin or fluorine-containing resin).

In order to avoid the invasion of moisture into the organic EL displaydevice, it is preferable to subject the substrate made of thesematerials to moisture proof treatment or hydrophobic treatment byforming an inorganic film further or applying a fluorine-containingresin.

Accordingly, in order to avoid the invasion of moisture into the organicluminescent medium, it is preferable to make the water content in thesubstrate and the gas permeability coefficient thereof small by moistureproofing treatment or hydrophobic treatment. Specifically, it ispreferable to set the water content in the supporting substrate and thegas permeability coefficient into 0.0001% by weight or less and 1×10⁻¹³or less cc·cm/cm²·sec.cmHg, respectively.

2. Organic EL Element

(1) Organic Luminescent Medium

The organic luminescent medium can be defined as a medium containing anorganic emitting layer wherein an electron and a hole are recombinedwith each other so that EL luminescence can be emitted. This organicluminescent medium can be constructed, for example, by laminating thefollowing respective layers on the bottom electrode. The triplet-relatedluminous compound may be contained in any of the following organiclayers.

-   (i) Organic emitting layer-   (ii) Hole transport layer/organic emitting layer-   (iii) Organic emitting layer/electron injection layer-   (iv) Hole transport layer/organic emitting layer/electron injection    layer-   (v) Hole transport layer/organic emitting layer/hole barrier    layer/electron injection layer-   (vi) Hole transport layer/electron barrier layer/organic emitting    layer/electron injection layer    1) Construction Material 1

Examples of the luminous material (host compound) in the organicluminescent medium include only one or combinations of two or moreselected from carbazole derivatives, p-quaterphenyl derivatives,p-quinquephenyl derivatives, benzothiazole compounds, benzimidazolecompounds, benzoxazole compounds, metal-chelated oxynoid compounds,oxadiazole compounds, styrylbenzene compounds, distyrylpyrazinederivatives, butadiene compounds, naphthalimide compounds, perylenederivatives, aldazine derivatives, pyraziline derivatives,cyclopentadiene derivatives, pyrrolopyrrole derivatives, styrylaminederivatives, coumarin compounds, aromatic dimethylidyne compounds, metalcomplexes each having an 8-quinolinol derivative as a ligand, andpolyphenyl compounds.

Among these host compounds, more preferable are4,4′-bis(2,2-di-t-butylphenylvinyl)biphenyl (abbreviated to DTBPBBi),4,4′-bis(2,2-diphenylvinyl)biphenyl (abbreviated to DPVBi), andderivatives thereof as the aromatic dimethylidyne compounds.

The host compound is preferably a compound having a larger singletexcitation energy than the level of the triplet excitation energyobtained by the triplet-related luminous compound so that the hostcompound can use the triplet excitation energy, and is more preferably acompound having a larger triplet excitation energy than the energylevel.

2) Construction Material 2

The phosphorescent luminous compound is a compound which generatesphosphorescence. It is sufficient that the phosphorescent luminouscompound is a compound which has an excitation state life span of 100 nsor more, and its life span can be measured as a luminescence relaxationcomponent. As the method of measuring the life span, the time dependentmeasurement of transit luminescent delay can be used.

The phosphorescent luminous compound is preferably a triplet-relatedcompound wherein a triplet state contributes to the step of emittingluminescence. It is particularly preferable to use a compound having atleast one metal selected from the group consisting of Ir, Pt, Pd, Ru,Rh, Mo, Re, Pb and Bi as a central metal or central metals, and a CNligand having a skeleton structure represented by the following formula(1), and a derivative thereof. Preferable is also a complex having mixedligands wherein one CN ligand or two CN ligands are coordinated andfurther one Lx (wherein Lx=OO, ON or the like) is coordinated. Examplesof the Lx ligand include acetylacetone derivatives (acac) and picolinederivatives (pic). Examples of such a triplet-related luminous compoundinclude only one or combinations of two or more selected from an iridiumcomplex, a platinum complex, a palladium complex, a ruthenium complex, arubidium complex, a molybdenum complex, and a rhenium complex.

Among these triplet-related luminous compounds, more specifically, thefollowing are more preferable: tris(2-phenylpyridyl)iridium (abbreviatedto Ir(ppy)₃), bis(2-phenylpyridyl)platinum,tris(2-phenylpyridyl)palladium, tris(2-phenylpyridyl)ruthenium,tris(2-phenylpyridyl)rubidium, tris(2-phenylpyridyl)molybdenum,Ir(ppy)₂(acac), Ir(btp)₂(acac), BtpPt(acac), Ir(bo)₂(acac),Ir(bt)₂(acac), Ir(ppy)₂(Pic), and Ir(btp)₂(pic).

This is because these triplet-related luminous compounds can giveluminescence property related to the triplet state even at roomtemperature.

These complexes may have a substituent. Examples of the substituentinclude alkyl groups, fluorine, and aryl groups.

It is preferable to make the ionization potential of the triplet-relatedluminous compound larger than the ionization potential of the hostcompound.

This is because such a triplet-related luminous compound causes a holetransferred from the anode to the organic emitting layer to beeffectively held inside the organic emitting layer so that theluminescence efficiency can be made higher.

The amount of the added triplet-related luminous compound is preferablyset into 0.1 to 50% by weight of the total amount of the organicluminescent medium.

This is because: if the amount of the added triplet-related luminouscompound is less than 0.1% by weight, the effect of the addition may notbe exhibited; and if the amount of the added triplet-related luminouscompound is more than 50% by weight, the half life may becomeexcessively short.

Accordingly, the amount of the added triplet-related luminous compoundis more preferably set into 1 to 30% by weight of the total amount ofthe organic luminescent medium, and still more preferably set into 5 to20% by weight thereof.

3) Construction Material 3

Examples of hole transport material which constitutes the hole transportlayer include only one or combinations of two or more selected fromtriazole derivatives, oxadiazole derivatives, imidazole derivatives,polyarylalkane derivatives, pyrazoline derivatives, pyrazolonederivatives, phenylenediamine derivatives, arylamine derivatives,amino-substituted chalcone derivatives, oxazole derivatives, fluorenonederivatives, hydrozone derivatives, styrylanthracene derivatives,stylbene derivatives, silazane derivatives, polysilane, aniline-basedcopolymers, and conductive high-molecular oligomer (in particular,thiophene oligomer).

More specifically, it is particularly preferable to use, among thesehole transport materials, a bis(diarylamino)arylene derivative whereinthe aryl is a polyphenyl and the arylene is a polyphenylene since thelife span thereof is particularly long.

The polyphenyl is preferably biphenyl or terphenyl, and thepolyphenylene is preferably biphenylyl or terphenylyl.

4) Construction Material 4

Examples of electron injection material which constitutes the electroninjection layer include only one or combinations of two or more selectedfrom tris(8-quinolinolate) aluminum, tris(8-quinolinolate) gallium,bis(10-benz[h]quinolinolate) beryllium, triazole derivatives, oxadiazolederivatives, triazine derivatives, perylene derivatives, quinolinederivatives, quinoxaline derivatives, diphenylquinone derivatives,nitro-substituted fluorenone derivatives, thiopyrandioxide derivatives.

It is also preferable to add the following as a dopant to this electroninjection material: an alkali metal, alkali earth metal, rare earthmetal, alkali compound, alkali earth compound, rare earth compound, oralkali metal to which an organic compound coordinates.

5) Thickness

The thickness of the organic luminescent medium is not particularlylimited. For example, the thickness is preferably set into 5 nm to 5 μm.

This is because: if the thickness of the organic luminescent medium isless than 5 nm, the luminescence brightness or durability may lower;whereas if the thickness of the organic luminescent medium is more than5 μm, the value of applying-voltage may become high.

Accordingly, the thickness of the organic luminescent medium is morepreferably set into 10 nm to 3 μm, and still more preferably set into 20nm to 1 μm.

(2) Electrodes

The top electrode and the bottom electrode are described hereinafter.The top electrode and the bottom electrode correspond to an anode layerand a cathode layer, respectively, and a cathode layer and an anodelayer, respectively, correspondingly to the structure of the organic ELelement.

1) Bottom Electrode

The bottom electrode corresponds to an anode layer or a cathode layer,correspondingly to the structure of the EL display device. For example,in the case that it corresponds to an anode layer, it is preferable touse a metal, alloy or electrically conductive compound which has a largework function (for example, 4.0 eV or more), or a mixture thereof.Specifically, it is preferable to use an electrode material, such asindium tin oxide, indium zinc oxide, strontium copper oxide, tin oxide,zinc oxide, gold, platinum or palladium, alone, or use a combination oftwo or more of these electrode materials.

By using these electrode materials, the bottom electrode which has auniform thickness can be formed by a method capable of forming a film ina dry state, such as vapor deposition, sputtering, ion plating, electronbeam vapor deposition, CVD (chemical vapor deposition), MOCVD (metaloxide chemical vapor deposition), or plasma CVD (plasma enhancedchemical vapor deposition).

Since EL luminescence has to be taken out from the side of the bottomelectrode in the present invention, it is necessary to make the bottomelectrode to be a transparent electrode. It is therefore preferable touse the above-mentioned electrode material to set the transmittance ofEL luminescence into 70% or more.

The film thickness of the bottom electrode is not particularly limited.For example, the thickness is preferably set into 10 to 1,000 nm, andmore preferably set into 10 to 200 nm.

This is because a sufficient electric conductivity is obtained andfurther a high transmittance of 70% or more is obtained about ELluminescence by setting the film thickness of the bottom electrode intoa value within such a range.

2) Top Electrode

Meanwhile, the top electrode also corresponds to an anode layer or acathode layer, correspondingly to the structure of the EL displaydevice. For example, in the case that it corresponds to a cathode layer,it is preferable to use a metal, alloy or electrically conductivecompound which has a smaller work function (for example, less than 4.0eV) than the anode layer, or a mixture or inclusion thereof.

Specifically, it is preferable to use an electrode material, such assodium, sodium-potassium alloy, cesium, magnesium, lithium,magnesium-silver alloy, aluminum, aluminum oxide, aluminum-lithiumalloy, indium, a rare earth metal, a mixture of these metals and anorganic luminescent medium material, or a mixture of these metals and anelectron injection layer material, alone, or use a combination of two ormore of these electrode materials.

The film thickness of the top electrode is not particularly limited,either. Specifically, the thickness is preferably set into 10 to 1,000nm, and more preferably set into 10 to 200 nm.

This is because a given sheet resistance and a good electric connectionreliability can be obtained by setting the film thickness of the topelectrode into a value within such a range.

As illustrated in FIG. 1, the top electrode 20 is preferably composed ofa main electrode 16, and an auxiliary electrode 18 made of the materialwith a lower resistivity.

Such a structure makes it possible to make the sheet resistance of thetop electrode 20 remarkably low. Hence, the density of electric currentflowing through the organic luminescent medium can be reduced. As aresult, the life span of the organic luminescent medium can be maderemarkably long.

3. Color Converting Medium

It is also preferable to lay a color converting medium on the luminoussurface of the organic EL element. Examples of such a color convertingmedium include a color filter, and a fluorescent film for emitting lightin color different from that of EL luminescence. A combination thereofis also preferable.

(1) Color Filter

The color filter is set up to decompose or filter light, therebyadjusting color or improving contrast, and is made as a dye layer madeof only a dye, or as a lamellar matter made by dissolving or dispersinga dye into a binder resin.

About the construction of the color filter, it is preferable to containdyes in blue, green and red. This is because a combination of such acolor filter with an organic EL element which emits white luminescencemakes it possible to obtain the three primary colors of light, blue,green and red, and display full color.

It is preferable to use printing or photolithography so as to patternthe color filter in the same manner as in the case of a fluorescencemedium.

(2) Fluorescent Medium

The fluorescent mediums in the organic EL display device have a functionof absorbing luminescence of the organic EL element to emit fluorescencehaving a longer wavelength, and are made as a layer which are separatelyarranged in a planar direction. Respective fluorescent medium pieces arepreferably arranged to correspond to luminous areas of the organic ELelement, for example, positions of cross portions of the bottomelectrode and the top electrode. Such a structure makes it possible thatwhen the organic emitting layer in the cross portions of the bottomelectrode and the top electrode emits luminescence, the respectivefluorescent medium pieces receive the light and then luminescence indifferent colors (wavelengths) can be taken out. In particular, such astructure that the organic EL element emits blue luminescence andfurther the luminescence can be converted to green or red luminescenceby the fluorescent medium makes it possible that the three primarycolors of light, blue, green and red are obtained even from the singleorganic EL element and full-color display is attained. Thus, thisstructure is preferable.

It is also preferable to arrange, between the respective fluorescentmedium pieces, a light shielding layer (black matrix) for blockingluminescence of the organic EL element and light from the respectivefluorescent medium pieces to improve contrast and decrease thedependency on field angle.

In order to prevent a fall in contrast by external light, thefluorescent medium may be constructed to be combined with theabove-mentioned color filter.

4. Driving Circuit

(1) Voltage

It is preferable to set the voltage value when voltage is applied (orelectric current is injected) into 1 to 20 V.

This is because: if this voltage value is less than 1 V, a desiredluminescence brightness is not obtained; whereas if this voltage valueis more than 20 V, consumption power may be large.

Accordingly, the voltage value when voltage is applied or electriccurrent is injected is preferably set into 3 to 15 V, and still morepreferably set into 8 to 13 V.

(2) Frequency

It is necessary that when voltage is applied or electric current isinjected, a pulse wave is used and the frequency thereof is set into 30Hz or more.

This is because if this frequency is less than 30 Hz, the resultant ELdisplay flickers. However, if the value of the frequency becomesexcessively large, a problem may be caused that deterioration of theorganic luminescent medium is promoted so that the luminous life spanfalls.

Accordingly, the frequency when voltage is applied or electric currentis injected is preferably set into 40 to 120 Hz, and more preferably setinto 50 to 100 Hz.

(3) Duty Ratio

It is also necessary to set the duty ratio (corresponding to t1/T inFIG. 5) of the pulse wave when voltage is applied (or electric currentis injected), into 1/5 or less.

This is because if this duty ratio is more than 1/5, the half life ofthe organic EL display device is short. However, if this duty ratiobecomes excessively small, a problem may be caused that the luminescencebrightness lowers.

Accordingly, the duty ratio when voltage is applied or electric currentis injected is preferably set into 1/1000 to 1/10, and more preferablyset into 1/500 to 1/20.

Referring to FIG. 2, relationship between the duty ratio and the halflife is described herein in more detail. As the transverse axis in FIG.2, the duty ratio (−) is taken and represented, and as the verticalaxis, the half life (Hrs) of the organic EL display devices of Example 1and others is taken and represented.

As is easily understood from FIG. 2, the half life tends to be shorteras the duty ratio is larger. The duty ratio changes largely within arange that the duty ratio is from 0.1 to 0.2. When the half life, whichis about 400 Hrs in the case that the duty ratio is 0.1, exceeds 0.2,the half life falls to about 200 Hrs. Conversely, if this duty ratio isset into a value of 1/5 or less, that is, a value of 0.2 or less, arelatively long half life can be obtained. If this duty ratio is setinto a value of 0.1 or less, a longer half life can be obtained but thevalue of the half life tends to be saturated.

Thus, the following conclusion can be obtained from FIG. 2, as well: inorder to obtain a long half life, the duty ratio is indispensably setinto 1/5 or less, preferably set into 1/1000 to 1/10, and morepreferably set into 1/500 to 1/20, as described above. This result is areason for daring to adopt the above-mentioned duty ratio in activedriving, which is originally DC-driven.

(4) Thin Film Transistor (TFT)

1) Structure

As illustrated in FIG. 1, the organic EL display device of the presentinvention preferably has at least one TFT 14 and an organic EL element26 driven by this TFT 14 on or over a supporting substrate 10.

That is, an interlayer insulating film 13 and a color converting medium60, which are made flat, are arranged between the TFT 14 and a bottomelectrode 22 of the organic EL element 26, and a drain 47 of the TFT 14is electrically connected to the bottom electrode 22 of the organic ELelement 26 through an electrically connecting member 28 placed in theboundary between the interlayer insulating film 13 and the colorconverting medium 60.

As illustrated in a circuit diagram of FIG. 3 including a TFT and alayout plan of FIG. 4 including the TFT, to the TFT 14 are electricallyconnected plural scan electrode lines (Yj to Yj+n) 50, (the number ofwhich is n, and n is, for example, from 1 to 1,000), and signalelectrode lines (Xi to Xi+n) 51, which are arranged in an XY matrixform. Furthermore, common electrode lines (Ci to Ci+n) 52 laid inparallel to the signal electrode lines 51 are electrically connected tothe TFT 14.

It is preferable that these electrode lines 50, 51 and 52 areelectrically connected to the TFT 14 and they are combined withcondensers 57 to constitute electric switches for driving the organic ELelements 26. In other words, it is preferable that the electric switchesare electrically connected to the scan electrode lines 50, the signalelectrode lines 51 and others, and are further composed of, for example,one or more first transistors (which may be referred to as Tr1hereinafter) 55, second transistors (which may be referred to as Tr2hereinafter) 56, and condensers 57.

It is preferable that the first transistor 55 has a function ofselecting a luminous pixel and the second transistor 56 has a functionof driving an organic EL element.

As illustrated in FIG. 1, an active layer 44 of the first transistor(Tr1) 55 and the second transistor (Tr2) 56 is composed of semiconductorareas 45 and 47 doped into n type and a non-doped semiconductor area 46,and can be represented by n+/i/n+.

The semiconductor areas doped into the n type become a source 45 and thedrain 47, respectively, and they are combined with a gate 43 provided tothe upper of the non-doped semiconductor area across a gate oxide film12 so as to constitute the transistors 55 and 56 as a whole.

In the active layer 44, the semiconductor areas 45 and 47, which aredoped into the n type, may be doped into p type to make a structure ofp+/i/p+. The active layer 44 of the first transistor (Tr1) 55 and thesecond transistor (Tr2) 56 is preferably made of an inorganicsemiconductor such as polysilicon, or an organic semiconductor such asthiophene oligomer or poly(p-phenylenevinylene). Polysilicon is aparticularly preferable material since it is more stable againstelectric conduction than amorphous Si(α-Si).

2) Driving Method

The following describes a method for driving organic EL elements basedon a TFT. It is preferable that this TFT includes the first transistor(Tr1) 55 and the second transistor (Tr2) 56 and further constitutes anelectric switch, as illustrated in FIG. 3.

That is, the electric switch having such a structure makes it possibleto input a scan signal pulse and a signal pulse through electrodes in anXY matrix form and perform switching operation, thereby driving theorganic EL element 26.

More specifically, through the electric switch, the organic EL element26 is caused to emit light or the light emission is stopped, whereby animage can be displayed.

When the organic EL elements 26 are driven through the electric switchesin this way, a desired one of the first transistors (Tr1) 55 is selectedby a scan pulse transmitted through the scan electrode lines (which maybe referred to as gate lines) (Yj to Yj+n) 50 and a scan pulsetransmitted through the signal electrode lines (Xi to Xi+n) 51, so thata given electric charge is collected in one of the condensers 57 formedbetween the common electrode lines (Ci to Ci+n) 52 and the source 45 ofthe first transistor (Tr1) 55.

In this way, the gate voltage of the second transistor (Tr2) 56 turns toa constant value, so that the second transistor (Tr2) 56 turns into anON-state. In this ON-state, the gate voltage is held until a next gatepulse is transmitted. Accordingly, electric current continues to besupplied to the bottom electrode 22 of the organic EL element 26connected to the drain 47 of the second transistor (Tr2) 56.

Consequently, the organic EL element 26 can be driven by the suppliedelectric current. Thus, the driving voltage of the organic EL element 26can be largely lowered and the luminescence efficiency can be improved.Moreover, the consumption power can be reduced.

5. Sealing Part

It is preferable to provide a sealing part to the organic EL displaydevice. It is preferable to set such a sealing part onto the surroundingof the organic EL display device in order to prevent the invasion ofmoisture therein, or to insert a known sealing medium, such as a dryingagent, a dry gas, or an inert gas such as fluorohydrocarbon, air-tightlybetween the sealing part set in this way and the organic EL displaydevice.

This sealing part can be used as a supporting substrate in the case thatthe fluorescent or the color filter is set outside the top electrode.

As this sealing part, the same material as constitutes the supportingsubstrate, for example, a glass plate can be used. A thin film layermade of oxide, oxynitride, nitride, sulfide or the like may also beused. Preferred examples of this material include SiO_(x) (1<x≦2),SiO_(x)N_(y) (1<x<2, and 0<y<1.5), AlO_(x) (0.6<x≦1.5), AlON, SiAlON,SiC, and SiCN. (x or y represents a composition ratio, and SiAlON or thelike, in which neither x nor y is indicated, represents any compositionratio). The form of the sealing part is not particularly limited. Theform is preferably made up, for example, to a plate form or a cap form.For example, when the form is made up to a plate form, the thicknessthereof is preferably set into 0.01 to 5 mm.

It is also preferable to make a groove or the like in a part of theorganic EL display device, and push and fix the sealing part into itunder pressure, or to use a photo-curable type adhesive agent or thelike to fix the sealing part onto a part of the organic EL displaydevice.

Second Embodiment

A second embodiment is an organic EL display device comprising anorganic EL element having a structure wherein an organic luminescentmedium is sandwiched between a top electrode and a bottom electrode, anda driving circuit for driving the organic EL element, wherein theorganic luminescent medium comprises a host compound and atriplet-related luminous compound; the driving circuit applies aelectric pulse voltage or pulse current having a frequency of 30 Hz ormore and a duty ratio of 1/5 or less; and further after the drivingcircuit applies the electric pulse voltage or pulse current to cause theorganic luminescent medium to emit luminescence, the driving circuitapplies a voltage (V2) in the direction reverse to that of the voltage(V1) of the pulse wave applied between the electrodes of the organic ELelement.

About the second embodiment, conditions for applying the reversedirection voltage, and others, which are different from those in thefirst embodiment, are mainly described hereinafter.

1. Reverse Voltage Value 1

It is preferable that after the driving circuit applies the electricpulse voltage or pulse current to the organic luminescent medium so asto cause the organic luminescent medium to emit luminescence, that is,when the electric pulse voltage or pulse current is not applied, thedriving circuit applies a voltage (V2) in the direction reverse to thatof the voltage (V1) of the pulse wave applied at the time of emittingthe luminescence. For example, in the case that a plus-direction voltageis applied at the time of emitting the luminescence, at the time of notapplying this voltage a minus-direction voltage is applied to theorganic luminescent medium.

By applying the voltage to the organic EL display device in this way,the consumption power can be made lower and the luminous life span canbe made longer even when the triplet-related luminous compound is used.That is, the triplet-related luminous compound has a problem thatelectric charge is liable to be stored therein and thus the innerelectric field intensity thereof changes with the passage of time andbalance between holes and electrons is broken, so that the luminescenceperformance thereof changes easily. Thus, by applying the reversevoltage in this way, the charge stored in the triplet-related luminouscompound can be removed.

Hence, by applying the reverse voltage to the organic luminescentmedium, the problem of inner storage of charge is overcome even if thetriplet-related luminous compound is used. As a result, the consumptionpower is made lower and the luminous life span can be made longer. Inthe case that electric current is applied at the time of emittingluminescence, the voltage necessary for giving the electric current isdefined as V1.

2. Reverse Voltage Value 2

It is also preferable to set the reverse voltage value when the electricpulse voltage or pulse current is not applied into 0.01 to 15 V.

This is because if this reverse voltage value is less than 0.01 V, theelectric charge stored in the triplet-related luminous compound isinsufficiently removed so that the effect of the application of thereverse voltage may not be obtained. On the other hand, if this reversevoltage value is more than 15 V, the organic luminescent medium may bedeteriorated or destroyed.

Accordingly, the reverse voltage value is preferably set into 0.1 to 10V, and more preferably set into 0.5 to 5 V.

For the application of the reverse voltage, a direct current and analternate current (which includes a pulse wave), or either one thereofcan be used. The use of a pulse wave is preferable since the electriccharge stored in the triplet-related luminous compound can beeffectively removed.

About the electric current, a direct current and an alternate current(which includes a pulse wave), or either one thereof can be used.

3. Reverse Voltage Value 3

It is preferable to decide the absolute value (V2) of the reversevoltage, considering the value of the voltage (V1) applied when theorganic luminescent medium emits luminescence. That is, the absolutevalue (V2) of the reverse voltage is preferably set into 1 to 90% of thevoltage (V1) applied when the organic luminescent medium emitsluminescence.

This is because if this absolute value of the reverse voltage is lessthan 1% of the V1, the electric charge stored in the triplet-relatedluminous compound is insufficiently removed so that the effect of theapplication of the reverse voltage may not be obtained.

On the other hand, if the absolute value of the reverse voltage is morethan 90% of the V1, the organic luminescent medium may be deterioratedor destroyed.

Accordingly, this absolute value of the reverse voltage is preferablyset into 5 to 80% of the V1, and more preferably set into 10 to 50%thereof.

4. Frequency

When the reverse voltage is applied, it is preferable to use a pulsewave, as well. In this case, the frequency of the pulse wave ispreferably set into 10 to 120 Hz.

This is because if this frequency is less than 10 Hz, a problem may becaused that the stored charge is insufficiently removed. On the otherhand, if this frequency is more than 120 Hz, deterioration of theorganic luminescent medium is promoted so that the luminous life spanmay decrease.

Accordingly, the frequency of the pulse wave of the reverse voltage ispreferably set into 20 to 100 Hz, and more preferably set into 30 to 80Hz.

5. Duty Ratio

The duty ratio of the pulse wave when the reverse voltage is applied ispreferably set into 1/20 to 1-the duty ratio of the pulse wave in theforward direction.

This is because if this duty ratio is less than 1/20, the charge storedin the triplet-related luminous compound is insufficiently removed sothat the effect of the application of the reverse voltage may not beobtained.

On the other hand, this duty ratio cannot be made larger than 1-the dutyratio of the pulse wave in the forward direction.

Accordingly, the duty ratio of the pulse wave when the reverse voltageis applied is preferably set into the above-mentioned range, and morepreferably set into 1/10 to 90/100.

6. Applying Timing of the Reverse Voltage

The timing when the reverse voltage is applied is any other time thantime when the electric pulse voltage or pulse current is applied tocause the organic EL element to emit luminescence. That is, if thetiming is any time when the voltage is not applied, no especial problemis caused whether the timing is any time when the organic EL elementcontinues to emit luminescence or any time when the EL element emits noluminescence.

It is however preferable to apply the reverse voltage in accordance withtiming charts shown in FIGS. 6 to 9 since the charge stored in thetriplet-related luminous compound can be effectively removed withoutpromoting deterioration of the organic luminescent medium.

That is, FIG. 6 shows that: at a time when a voltage is applied in orderto cause the organic EL element to emit luminescence and subsequently atime t2 passes, a reverse voltage is applied by a pulse wave for a timet3; and when a time t4 passes further, a voltage is again applied. FIG.7 shows that over a time t5 when the voltage is not applied to theorganic EL element, a DC reverse voltage is applied.

FIG. 8 shows that when the voltage is not applied to the organic ELelement, a reverse voltage is applied plural times by a pulse wave for atime t6, a time t7 and a time t8. In the example shown in FIG. 8, thevalue of the reverse voltage is made higher as time passes. A fear thatthe organic luminescent medium is damaged by the reverse voltage becomessmall.

Furthermore, FIG. 9 shows that when the voltage is not applied to theorganic EL element, a reverse voltage of an alternating wave is applied.It is also preferable to combine this alternating wave with theabove-mentioned pulse wave appropriately, which is not illustrated.

It is particularly preferable to set the time t2 to not less than 0 andnot more than the luminous life span of the triplet-related luminouscompound. This is particularly effective for removing the stored charge.

Third Embodiment

A third embodiment is an organic EL display device comprising an organicEL element having a structure wherein an organic luminescent medium issandwiched between a top electrode and a bottom electrode, and a drivingcircuit for driving the organic EL element, wherein the organicluminescent medium comprises a host compound and a triplet-relatedluminous compound; the driving circuit applies a electric pulse voltageor pulse current having a frequency of 30 Hz or more and a duty ratio of1/5 or less; and further a hole barrier layer is arranged between theorganic luminescent medium and the cathode.

The hole barrier layer, which is different from the first and secondembodiments, is mainly described hereinafter.

1. Kind

It is preferable to use, as the compound which constitutes the holebarrier layer, a compound having a larger ionization potential than theorganic emitting layer. In the present invention, it has been recognizedthat elements having the hole barrier layer particularly have theadvantageous effect for removing stored charge. This is because theadvantageous effect of the present invention is easily produced sincecharge is stored in the interface between the organic emitting layer andthe hole barrier layer. Examples of compounds, which constitutes suchhole barrier layer include phenanthroline derivatives represented by thefollowing formulae (2) to (5):

[In each of the formulae, R¹ to R¹⁰ represent a hydrogen, a halogenatom, a hydroxyl group, NO₂, CN, or a substituted or unsubstitutedalkyl, aryl or amino group.]

Other preferable examples thereof are metal complexes having an8-hydroxyquinoline derivative as a ligand. Particularly preferable arethe metal complexes having an energy gap of 2.8 eV or more.

2. Ionization Potential

In order to exhibit excellent hole barrier property, it is preferable tomake the ionization potential of the hole barrier layer larger than thatof the organic luminescent medium.

In order to use a triplet-related luminous compound in the organicemitting layer to improve the luminescence efficiency, it isparticularly preferable to make the ionization potential of the holebarrier layer 0.1 to 1 eV larger than that of the organic luminescentmedium.

3. Thickness

The thickness of the hole barrier layer is not particularly limited. Forexample, the thickness is preferably set into 1 nm to 1 μm.

This is because: if the thickness of the hole barrier layer is less than1 nm, the luminescence brightness or durability may lower; and if thethickness of the hole barrier layer is more than 1 μm, the value ofapplying-voltage may become high.

Accordingly, the thickness of the hole barrier layer is more preferably3 nm to 500 nm, and still more preferably 5 nm to 100 nm.

4. Formation Method

The method for forming the hole barrier layer is not particularlylimited. It is preferable to form the layer by a method such as spincoating, casting or screen printing, or form the layer by a method suchas sputtering, vapor deposition, chemical vapor deposition (CVD), or ionplating.

EXAMPLES Example 1

(1) Formation of an Organic EL Element

1) Formation of an Anode (Bottom Electrode)

An ITO film 130 nm in film thickness was formed on the whole of a glasssubstrate 112 mm long, 143 mm wide and 1.1 mm thick (OA2 glass,manufactured by Nippon Electric glass Co., Ltd.) by sputtering. Apositive resist HRP 204 (manufactured by Fuji Hunt ElectronicsTechnology Co., Ltd.) was applied onto this ITO film by spin coating,and the resultant was dried at a temperature of 80° C. for 15 minutes.

Next, contact exposure using a high-pressure mercury lamp as a lightsource was performed through a photo mask having a stripe-like pattern(line width: 90 μm, and gap width: 20 μm) in such a manner that thequantity of the exposure would be 100 mJ/cm². TMAH (tetramethylammoniumhydroxide) was used as a developing solution to perform development.

Next, an oven was used to conduct post baking treatment at a temperatureof 130° C. and subsequently an aqueous solution of hydrobromic acid(concentration: 47% by weight) was used as an etchant to etch the ITOfilm. Thereafter, a peeling solution N303 (manufactured by Nagase & Co.,Ltd.) was used to remove the positive resist, thereby forming an ITOpattern in a stripe form (the number of lines: 960) as an anode (bottomelectrode).

2) Formation of a First Interlayer Insulating Film

Next, an acrylic acid type negative resist V259 PA (manufactured byNippon Steal Chemical Co., Ltd.) was applied onto the ITO pattern byspin coating, and the resultant was dried at a temperature of 80° C. for15 minutes. Thereafter, contact exposure using a high-pressure mercurylamp as a light source was performed (exposure quantity: 300 mJ/cm²)through a photo mask allowing the ITO to be exposed in the form of 70μm×290 μm rectangles.

Next, TMAH was used as a developing solution to perform development, andfurther an oven was used to conduct post baking treatment at atemperature of 160° C. to form a first inter-insulator.

3) Formation of a Second Inter-Insulator

Next, a Novolak resin type negative resist ZPN 1100 (manufactured byNippon Zeon Co., Ltd.) was applied onto the first inter-insulator byspin coating. The resultant was dried at a temperature of 80° C. for 15minutes, and then contact exposure using a high-pressure mercury lamp asa light source was performed (exposure quantity: 70 mJ/cm²) through aphoto mask giving a stripe-like pattern (line width: 20 μm, and gapwidth: 310 μm) crossing the ITO pattern, which was the bottom pattern.Next, the resultant was baked at a temperature of 90° C. for 15 minutes.

Next, TMAH was used as a developing solution to perform development,thereby forming a second inter-insulator (line width: 20 μm, gap width:310 μm, and film thickness: 5 μm) as partition walls.

4) Dehydrating Step

Next, the glass substrate on which the ITO pattern and so on wereformed, (which may be referred to merely as the glass substratehereinafter), was washed with isopropyl alcohol and with ultravioletrays, and subsequently this glass substrate was shifted to a dehydratingunit for carrying out a dehydrating step. That is, the glass substratewas put into a dry box provided with an inert gas (nitrogen) circulatingsection, a dew point controlling section, and a heating device section(hot plate).

The hot plate was used to heat the glass substrate inside the dry box upto 60° C., and in this state dry nitrogen was introduced into the boxwhile the dew point was lowered to −50° C. The glass substrate wasallowed to stand still for about 2 hours, thereby removing moisture inthe first and second inter-insulators and moisture adhering to thesurface of the glass substrate and others.

5) Formation of an Organic Luminescent Medium

The heating of the hot plate was stopped so that the temperature of theglass substrate lowered to room temperature. Thereafter, the dew pointwas kept without exposing the glass substrate to the atmosphere and theglass substrate was fixed onto a substrate holder inside a vacuumdeposition machine (manufactured by ULVAC Japan, Ltd.).

The following materials were charged into a heating board made ofmolybdenum inside the vacuum deposition machine.

-   Hole transport material:    4,4′-bis[N-(1-naphthyl)-N-phenylamino]-biphenyl (abbreviated to NPD    hereinafter)-   Organic luminous material: 4,4′-N,N′-dicarbazolebiphenyl    (abbreviated to CBP hereinafter)/tris(2-phenylpyridyl)iridium    (abbreviated to Ir(Ppy)₃ hereinafter, content by percentage: 8 wt.    %)-   Electron injection material: tris(8-quinolinol)aluminum (abbreviated    to Alq hereinafter) Counter electrode (cathode): Al

Next, the vacuum degree of the vacuum deposition machine was reduced to665×10⁻⁷ Pa (5×10⁻⁷ Torr), and layers were laminated by vacuum drawingone time without breaking the vacuum state in the middle from theformation of a hole transport layer to the formation of a cathode, so asto form an organic luminescent medium and so on so that a vapordeposition rate and film thickness were as follows:

-   NPD: a vapor deposition rate of 0.1 to 0.3 nm/second, and a film    thickness of 50 nm-   CBP·Ir(Ppy)₃: a vapor deposition rate of 0.1 to 0.3 nm/second, and a    total film thickness of 40 nm (CBP and Ir(Ppy)₃ were mixed and    evaporated.)-   Alq: a vapor deposition rate of 0.1 to 0.3 nm/second, and a film    thickness of 20 nm-   Alq·Li: a vapor deposition rate of 0.5 to 1.0 nm/second, and a total    film thickness of 10 nm (Alq and Li were mixed and evaporated.)-   Al: a vapor deposition rate of 0.5 to 1.0 nm/second, and a film    thickness of 150 nm    6) Sealing Step

Next, a sealing glass substrate (blue sheet glass, manufactured byGeomatec Co., Ltd.) was laminated on the cathode inside a sealing unitin which dry nitrogen was introduced, and the surrounding thereof wasair-tightly covered with a photo-curable type adhesive agent TB 3102(manufactured by Three Bond Co., Ltd.) to obtain an organic EL displaydevice for measuring luminescence performance.

(2) Evaluation of the Organic EL Element

A pulse current having a frequency of 60 Hz, a duty ratio of 1/10 and acurrent value of 24 mA/cm² was applied, from a driving circuit, betweenthe bottom electrode (ITO pattern, anode) of the resultant organic ELdisplay device and the top electrode (cathode) thereof, which was thecounter electrode, so as to cause respective pixels (about 230000pixels), which corresponded to cross points between the electrodes, toemit luminescence. A chroma meter CS 100 (manufactured by Minolta Co.,Ltd.) was used to measure the luminescence brightness. As a result, avalue of 500 cd/m² was obtained. The voltage necessary for giving thepulse current at this time was 10 V.

Under the same conditions, the respective pixels of the organic ELdevice were caused to emit luminescence, and the CIE chromaticitythereof was measured. As a result, it was proved that blue luminescencewherein CIEx=0.30 and CIEy=0.63 in CIE chromaticity coordinates wasobtained.

Next, the resultant organic EL display device was left in the atmosphereat room temperature (25° C.), and continuously driven to measure thehalf life. As a result, the half life was 400 hours.

In conclusion, even if the organic luminescent medium is doped withIr(Ppy)₃, which is a triplet-related luminous compound, the displaydevice can be driven by a voltage of 10 V or less to make consumptionpower low and further the luminous life span can be made long by usingthe specific driving circuit.

Example 2

An organic EL element was formed and then a pulse current having afrequency of 60 Hz, a duty ratio of 1/10 and a current value of 20mA/cm² was applied thereto from the driving circuit to make anevaluation in the same way as in Example 1 except that a hole barrierlayer (film thickness: 10 nm) made of2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline was laid between theluminous layer and the electron injection layer of the organic ELdisplay device in Example 1.

As a result, the luminous brightness was 500 cd/m², and the CIEchromaticity was CIEx=0.30 and CIEy=0.63. Furthermore, the half life was400 hours.

That is, it was proved that by laying the hole barrier layer, the halflife, which was 400 hours, did not change but the peak of the voltagewas lowered to 8 V to reduce consumption power.

Example 3

The organic EL element was evaluated in the same way as in Example 2except that the duty ratio of the driving voltage in Example 2 waschanged from 1/10 to 1/100, and a pulse current having a frequency of 60Hz and a current value of 410 mA/cm² was applied from the drivingcircuit.

As a result, the luminous brightness was 500 cd/m², and the CIEchromaticity was CIEx=0.30 and CIEy=0.63. Furthermore, the half life was400 hours.

That is, by making the duty ratio small, the current valueinstantaneously became far larger than that in Example 2 but the halflife, which was 400 hours, did not change. It is known that in drivewherein the current value instantaneously becomes large so that the dutyratio is large, the life span usually becomes short. However, it wasproved that the life span can be kept.

Example 4

The duty ratio of the same driving voltage as in the element in Example2 was changed from 1/10 to 1/100 and the frequency was changed from 60Hz to 500 Hz. Furthermore, the organic EL element was evaluated in thesame way as in Example 2 except that a pulse voltage (peak voltage: 14V) was applied.

As a result, the luminous brightness was 500 cd/m², and the CIEchromaticity was CIEx=0.30 and CIEy=0.63. Furthermore, the half life was460 hours.

That is, it was proved that even when the duty ratio was made small, thehalf life became 15% larger than that in Examples 1 and 2 by making thefrequency large.

Example 5

The organic EL element was evaluated in the same way as in Example 2except that a reverse voltage of 1 V was applied when no luminescencewas emitted, and a pulse current having a frequency of 60 Hz, a dutyratio of 1/10 and a current value of 20 mA/cm² was applied from thedriving circuit.

As a result, the luminous brightness was 500 cd/m², and the CIEchromaticity was CIEx=0.30 and CIEy=0.63. Furthermore, the half life was600 hours.

That is, it was proved that the half life became 50% larger than that inExamples 1 and 2 by applying the reverse voltage.

Example 6

The organic EL element was evaluated in the same way as in Example 2except that the duty ratio of the driving voltage in Example 2 waschanged from 1/10 to 1/7, and a pulse current having a frequency of 60Hz and a current value of 12 mA/cm² was applied from the drivingcircuit.

As a result, the luminous brightness was 500 cd/m², and the CIEchromaticity was CIEx=0.30 and CIEy=0.63. Furthermore, the half life was280 hours.

That is, it was proved that the current value was lowered and the halflife was lowered by making the duty ratio somewhat large.

Example 7

An organic EL element was formed and then a pulse current having afrequency of 60 Hz, a duty ratio of 1/10 and a current value of 18.5mA/cm² was applied thereto from the driving circuit for evaluation inthe same way as in Example 2 except that instead of NPD of the holetransport material in Example 2, a bis(arylamino)biphenyl derivativerepresented by the following formula (6) was used:

As a result, the luminous brightness was 500 cd/m², and the CIEchromaticity was CIEx=0.30 and CIEy=0.63. Furthermore, the half life was3,000 hours.

In conclusion, it was proved that the half life can be increased verylargely by changing the kind of the organic luminescent medium.

Comparative Example 1

In Example 2, a constant current value of 1.2 mA/cm² was applied fromthe driving circuit for evaluation.

As a result, the luminous brightness was 500 cd/m², and the CIEchromaticity was CIEx=0.30 and CIEy=0.63. However, the half life was 200hours, which were about 50% of that in Examples 1 and 2.

In conclusion, it was proved that when the duty ratio becomesexcessively large (D=1/1), the half life becomes very lowcorrespondingly.

Comparative Example 2

In Example 2, a pulse current having a frequency of 50 Hz, a duty ratioof 1/4 and a current value of 5.8 mA/cm² was applied from the drivingcircuit for evaluation.

As a result, the luminous brightness was 500 cd/m², and the CIEchromaticity was CIEx=0.30 and CIEy=0.63. However, the half life was 200hours, which were about 50% of that in Examples 1 and 2.

In conclusion, it was proved that when the duty ratio becomesexcessively large (D=1/4), the half life becomes very lowcorrespondingly.

Industrial Applicability

According to the organic EL display device of the present invention,even when a triplet-related luminous compound is used in its organicluminescent medium, consumption power can be made low and further theluminous life span thereof can be made long.

According to the method for driving an organic EL display device of thepresent invention, even when an organic EL display device using, in itsorganic luminescent medium, a triplet-related luminous compound iscaused to emit luminescence, consumption power can be made low andfurther the luminous life span thereof can be made long.

1. An organic electroluminescence display device, comprising: an organicelectroluminescence element having a structure wherein an organicluminescent medium comprising a host compound and a phosphorescentluminous compound is sandwiched between a top electrode and a bottomelectrode; and a driving circuit for applying a electric pulse currentor pulse voltage having a frequency of 30 Hz or more and a duty ratio of1/5 or less to drive the organic electroluminescence element.
 2. Theorganic electroluminescence display device according to claim 1, whereinthe driving circuit comprises a thin film transistor for controlling theluminescence of the organic electroluminescence element.
 3. The organicelectroluminescence display device according to claim 1, wherein thedriving circuit applies the electric pulse voltage or pulse current tocause the organic luminescent medium to emit luminescence, andsubsequently applies a voltage (V2) in a direction reverse to that ofthe voltage (V1) of the pulse wave applied between the electrodes of theorganic electroluminescence element.
 4. The organic electroluminescencedisplay device according to claim 3, wherein the driving circuit appliesthe voltage (V2) which is smaller than the voltage (V1) of the pulsewave and is in the direction reverse to that of the voltage (V1).
 5. Theorganic electroluminescence display device according to claim 1, furthercomprising a hole barrier layer between the organic luminescent mediumand the cathode.
 6. The organic electroluminescence display deviceaccording to claim 5, wherein the hole barrier layer comprises aphenanthroline derivative.
 7. The organic electroluminescence displaydevice according to claim 1, wherein the phosphorescent luminouscompound has a light emitting property to which a triplet statecontributes.
 8. The organic electroluminescence display device accordingto claim 7, wherein the phosphorescent luminous compound having thelight emitting property to which the triplet state contributes is anorganic metal complex.
 9. The organic electroluminescence display deviceaccording to claim 8, wherein the organic metal complex comprises atleast one metal selected from the group consisting of Ir, Pt, Pd, Ru,Rh, Mo, Re, Pb and Bi.
 10. A method for driving an organicelectroluminescence display device comprising an organicelectroluminescence element having a structure wherein an organicluminescent medium containing a phosphorescent luminous compound issandwiched between a top electrode and a bottom electrode, comprising:applying an electric pulse current or pulse voltage having a frequencyof 30 Hz or more and a duty ratio of 1/5 or less by a driving circuit,to drive the organic electroluminescence element.
 11. The method fordriving the organic electroluminescence display device according toclaim 10, wherein the driving circuit applies the electric pulse voltageor pulse current to cause the organic luminescent medium to emitluminescence, and subsequently applies a voltage (V2) in a directionreverse to that of the voltage (V1) of the pulse wave applied betweenthe electrodes of the organic electroluminescence element.